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Number theory Difficulty 7.9 National Olympiad, round 2 Prove it Germany

Problem:

Show that there exists an integer aa for which a336a2+51a97a^{3}-36 a^{2}+51 a-97 is a multiple of 320083^{2008}.

Solution

Solution:

1st Solution: Define the polynomial P(x)=x336x2+51x97P(x)=x^{3}-36 x^{2}+51 x-97. Starting from P0(x)=181P(9x+1)=9x333x22x1P_{0}(x)=\frac{1}{81} P(9 x+1)=9 x^{3}-33 x^{2}-2 x-1, define recursively polynomials Pk(x)=3akx3+3bkx2+ckx+dkP_{k}(x)=3 a_{k} x^{3}+3 b_{k} x^{2}+c_{k} x+d_{k} with integers ak,bk,ck,dka_{k}, b_{k}, c_{k}, d_{k}, where ckc_{k} is not divisible by 3, and Pk+1(x)=13Pk(3x+εk)P_{k+1}(x)=\frac{1}{3} P_{k}\left(3 x+\varepsilon_{k}\right) with εk{1,0,1}\varepsilon_{k} \in\{-1,0,1\} for all kk: Since ckc_{k} is not divisible by 3, one can choose εk\varepsilon_{k} such that εkck+dk\varepsilon_{k} c_{k}+d_{k} is divisible by 3; then 13Pk(3x+εk)=3ak+1x3+3bk+1x2+ck+1x+dk+1\frac{1}{3} P_{k}\left(3 x+\varepsilon_{k}\right)=3 a_{k+1} x^{3}+3 b_{k+1} x^{2}+c_{k+1} x+d_{k+1} with ak+1=9ak,bk+1=9εkak+3bka_{k+1}=9 a_{k}, b_{k+1}=9 \varepsilon_{k} a_{k}+3 b_{k}, ck+1=3(3εk2ak+2εkbk)+ck,dk+1=εk3ak+εk2bk+(εkck+dk)/3c_{k+1}=3\left(3 \varepsilon_{k}^{2} a_{k}+2 \varepsilon_{k} b_{k}\right)+c_{k}, d_{k+1}=\varepsilon_{k}^{3} a_{k}+\varepsilon_{k}^{2} b_{k}+\left(\varepsilon_{k} c_{k}+d_{k}\right) / 3, so ck+1c_{k+1} is also not divisible by 3.
Setting x2004=0x_{2004}=0 and recursively xk=3xk+1+εkx_{k}=3 x_{k+1}+\varepsilon_{k}, i.e. Pk(xk)=3Pk(xk+1)P_{k}\left(x_{k}\right)=3 P_{k}\left(x_{k+1}\right) for k=2003,,0k=2003, \ldots, 0, then for a=9x0+1a=9 x_{0}+1 the number P(a)=8132004P2004(x2004)P(a)=81 \cdot 3^{2004} P_{2004}\left(x_{2004}\right) is a multiple of 320083^{2008}.

2nd Solution: Define the polynomial P(x)=x336x2+51x97P(x)=x^{3}-36 x^{2}+51 x-97. For k4k \geq 4, integers aka_{k} with ak1mod9a_{k} \equiv 1 \bmod 9 and 3kP(ak)3^{k} \mid P\left(a_{k}\right) are constructed inductively; the problem statement is then satisfied by a=a2008a=a_{2008}:
Base case k=4k=4: For a4=1a_{4}=1 we have 34P(a4)3^{4} \mid P\left(a_{4}\right).
Induction step kk+1k \rightarrow k+1: For k4k \geq 4 we have 2k3k+12 k-3 \geq k+1 and 3k6>k+13 k-6>k+1, so for
all integer xx:

P(x+3k2)P(x)3k1x2+32k3x+33k636(23k2x+32k4)+513k23k1(x224x+17)mod3k+1 \begin{aligned} P\left(x+3^{k-2}\right)-P(x) & \equiv 3^{k-1} x^{2}+3^{2 k-3} x+3^{3 k-6}-36\left(2 \cdot 3^{k-2} x+3^{2 k-4}\right)+51 \cdot 3^{k-2} \\ & \equiv 3^{k-1}\left(x^{2}-24 x+17\right) \bmod 3^{k+1} \end{aligned}

For x1mod9x \equiv 1 \bmod 9 we have x224x+17124+173mod9x^{2}-24 x+17 \equiv 1-24+17 \equiv 3 \bmod 9. Because k4k \geq 4 we also have xx+3k2mod9x \equiv x+3^{k-2} \bmod 9. Therefore the difference 3k1(x224x+17)3^{k-1}\left(x^{2}-24 x+17\right) contains the prime factor 3 exactly kk times, so P(u)P(u) is divisible by 3k+13^{k+1} for u=aku=a_{k} or u=ak+3k2u=a_{k}+3^{k-2} or u=ak+23k2u=a_{k}+2 \cdot 3^{k-2}; set ak+1a_{k+1} to this value of uu. By construction ak+11mod9a_{k+1} \equiv 1 \bmod 9.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from de; metadata (topic, difficulty) added by this project.